The present invention relates to a method of making an optical fiber preform from which an optical fiber can be obtained by drawing.
Dispersion-shifted optical fibers have a zero-dispersion wavelength, where the wavelength dispersion value becomes zero, in the vicinity of 1.55 xcexcm; one kind of which has a ring core region with a high refractive index disposed around a center core region, and a cladding region disposed around the ring core region. A dispersion-shifted optical fiber having such a refractive index profile of a ring type structure is manufactured by drawing an optical fiber preform having a similar refractive index profile.
Such a refractive index profile of an optical fiber preform can be realized by the center core portion made of silica glass which is to become the center core region of the optical fiber and the ring core portion made of silica glass including GeO2 (germanium dioxide) which is to become the ring core region of the optical fiber. In general, the optical fiber preform is manufactured by VAD (vapour-phase axial deposition) method, OVD (outside vapour deposition) method, or the like.
When the optical fiber preform having the above-mentioned refractive index profile is manufactured by VAD method or OVD method, while gases for doping F element such as CF4 gas, SiF4 gas, and the like are introduced so as to add F element to the center core portion, F element is also doped in the ring core portion. If F element is doped in the ring core portion which should have a high refractive index, the overall refractive index will decrease, so that a desirable refractive index profile cannot be attained, whereby desirable fiber characteristics cannot be obtained. If the GeO2 concentration in the ring core portion is enhanced as measures against this problem, the transmission loss in the optical fiber will increase, and nonlinear optical phenomena will be more likely to occur.
In order to eliminate the foregoing problems, it is an object of the present invention to provide a method of making an optical fiber preform which can favorably make an optical fiber preform having a refractive index profile of a ring type structure whose center core portion is doped with F element.
The optical fiber making method in accordance with the present invention comprises: (1) an insertion step of inserting a silica glass rod having a cylindrical form doped with F element inside a tubular silica glass pipe including a region with a refractive index greater than that of pure silica glass; and (2) a heat-collapsing step of heating and collapsing the silica glass pipe and silica glass rod which are brought into an inserted state by the insertion step.
According to this optical fiber making method, while a desirable amount of F element is doped in the silica glass rod which is to become the center core region having a low refractive index in the optical fiber, no F element is doped in the silica glass pipe which is to become the ring core region having a high refractive index in the optical fiber, whereby an optical fiber preform having a refractive index profile of a desirable ring type structure can easily be manufactured.
Preferable as the silica glass pipe in the optical fiber making method in accordance with the present invention is one prepared by: (a) a soot body synthesizing step of synthesizing a soot body of GeO2 and SiO2 on an outer peripheral surface of a starting pipe; (b) a transparent glass forming step of dehydrating and consolidating the soot body on the outer peripheral surface of the starting pipe so as to form a transparent glass body; and (c) a removing step of removing the portion of the starting pipe in the transparent glass body so as to prepare the silica glass pipe. Alternatively, preferable is the silica glass pipe prepared by: (a) a soot body synthesizing step of synthesizing a rod-shaped soot body of GeO2 and SiO2; (b) a transparent glass forming step of dehydrating and consolidating the soot body so as to form a transparent glass body; and (c) boring step of boring the transparent glass body along a center axis thereof so as to prepare the silica glass pipe. Also, preferable is the silica glass pipe prepared by: (a) a soot body synthesizing step of synthesizing a soot body of GeO2 and SiO2 on an outer peripheral surface of a starting rod; (b) a transparent glass forming step of dehydrating and consolidating the soot body so as to form a transparent glass body; and (c) a removing step of removing the portion of the starting rod in the transparent glass body so as to prepare the silica glass pipe. Further, preferable is the silica glass pipe prepared by: (a) a soot body synthesizing step of synthesizing a rod-shaped soot body of GeO2 and SiO2; (b) a transparent glass forming step of dehydrating and consolidating the soot body so as to form a transparent glass body; and (c) a pipe forming step of heating the transparent glass body to a softening temperature or higher and inserting a rod having a melting point higher than the softening temperature into the transparent glass body along the center axis thereof so as to prepare the silica glass pipe. According to any of these preparing methods, a silica glass pipe, doped with GeO2, having a refractive index higher than that of pure silica glass is prepared.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.